Several of the extensions involve measurements of sound intensity in a standing wave of sound. For these investigations you will need a cardboard tube an inch or more in diameter, your Sound Level Meter, and the tone generator you built in the Core Project for a sound source.
A. WHY IS A COAXIAL CABLE NEEDED?Every wire is potentially an antenna. Electromagnetic radiation can produce stray voltages in unshielded wires. The 60-Hz signal produced by the 120-volt service in most walls can be especially troublesome. Coaxial cable protects the small, unamplified signal from an electret microphone from stray electromagnetic interference.
B. SOLDERING COAXIAL CABLE TO A MICROPHONE.
C. SOLDERING THE COAXIAL CABLE TO SOLID CORE WIRES.
At the other end, the shielding must be attached to a
solid core wire so that it can be inserted into a
breadboard. If the shielded wire is stranded, it must
also be attached to solid core wire. This procedure is
almost identical to the procedure just completed.
Substitute short lengths of hook-up wire for the
microphone pins. Make sure that the insulation on the ground
(black) wire overlaps the insulation on the coaxial (gray)
cable. Then both wires can be brought together and
covered with electrical tape without creating a short
Attach the microphone and part of the coaxial cable to a metal rod so it can be moved inside the cardboard tube. Some suggestions are give below.
Adding sound to the interference tube is the easiest of all! Tape a speaker to one end of the cardboard tube. If there is a mismatch between the diameter of the speaker and tube, create a cardboard donut with inside and outside diameters that match your components. Then tape the speaker to the donut and the donut to the tube. Attach your Tone Generator to the speaker.
To change the effective length of the interference tube, create a movable, cardboard disk that closes the other end of your cardboard tube.
Below is a diagram of how the sound level meter, piston, and interference tube are combined to create the apparatus for investigating standing waves. You are ready to begin.
1. Move the microphone and the plunger as close to the speaker as practical. Keeping the microphone and the plunger together, pull them down the length of the tube. Note the change in the sound intensity (measured in volts on your multimeter) as you do this. If all is well, the voltages should rise and fall as the microphone moves down the tube. Stop testing when the plunger is close to the far end of the cardboard tube.